How do I smooth out two objects (with low poly) that have been unioned in blender 2.8?
You can join two spheres this way and adjust for your character as the process should be the same.

Create a sphere with 16 segments and 8 rings



Create a second sphere half the size



With the smaller sphere selected go into edit mode and line up the edge loop with the larger sphere. You can do this by eye



Add a boolean union to the large sphere and select the small sphere to join



Apply the boolean and shade smooth with a subdivision surface x2



To transform the body I would add a lattice with a vertex group on the base mesh to limit the lattice influence and this way you keep the head round



you can then transform the body shape as required